Use a static class as a unit of
organization for methods not
associated with particular objects.
Also, a static class can make your
implementation simpler and faster
because you do not have to create an
object in order to call its methods.
It is useful to organize the methods
inside the class in a meaningful way,
such as the methods of the Math class
in the System namespace.

To me, that example doesn't seem to cover very many possible usage scenarios for static classes. In the past I've used static classes for stateless suites of related functions, but that's about it. So, under what circumstances should (and shouldn't) a class be declared static?

12 Answers
12

I used to love utility classes filled up with static methods. They made a great consolidation of helper methods that would otherwise lie around causing redundancy and maintenance hell. They're very easy to use, no instantiation, no disposal, just fire'n'forget. I guess this was my first unwitting attempt at creating a service oriented architecture - lots of stateless services that just did their job and nothing else. As a system grows however, dragons be coming.

Polymorphism

Say we have the method UtilityClass.SomeMethod that happily buzzes along. Suddenly we need to change the functionality slightly. Most of the functionality is the same, but we have to change a couple of parts nonetheless. Had it not been a static method, we could make a derivate class and change the method contents as needed. As it's a static method, we can't. Sure, if we just need to add functionality either before or after the old method, we can create a new class and call the old one inside of it - but that's just gross.

Interface woes

Static methods cannot be defined through interfaces for logic reasons. And since we can't override static methods, static classes are useless when we need to pass them around by their interface. This renders us unable to use static classes as part of a strategy pattern. We might patch some issues up by passing delegates instead of interfaces.

Testing

This basically goes hand in hand with the interface woes mentioned above. As our ability of interchanging implementations is very limited, we'll also have trouble replacing production code with test code. Again, we can wrap them up but it'll require us to change large parts of our code just to be able to accept wrappers instead of the actual objects.

Fosters blobs

As static methods are usually used as utility methods and utility methods usually will have different purposes, we'll quickly end up with a large class filled up with non-coherent functionality - ideally, each class should have a single purpose within the system. I'd much rather have a five times the classes as long as their purposes are well defined.

Parameter creep

To begin with, that little cute and innocent static method might take a single parameter. As functionality grows, a couple of new parameters are added. Soon further parameters are added that are optional, so we create overloads of the method (or just add default values, in languages that support them). Before long, we have a method that takes 10 parameters. Only the first three are really required, parameters 4-7 are optional. But if parameter 6 is specified, 7-9 are required to be filled in as well... Had we created a class with the single purpose of doing what this static method did, we could solve this by taking in the required parameters in the constructor, and allowing the user to set optional values through properties, or methods to set multiple interdependent values at the same time. Also, if a method has grown to this amount of complexity, it most likely needs to be in its own class anyways.

Demanding consumers to create an instance of classes for no reason

One of the most common arguments is, why demand that consumers of our class create an instance for invoking this single method, while having no use for the instance afterwards? Creating an instance of a class is a very very cheap operation in most languages, so speed is not an issue. Adding an extra line of code to the consumer is a low cost for laying the foundation of a much more maintainable solution in the future. And finally, if you want to avoid creating instances, simply create a singleton wrapper of your class that allows for easy reuse - although this does make the requirement that your class is stateless. If it's not stateless, you can still create static wrapper methods that handle everything, while still giving you all the benefits in the long run. Finally, you could also make a class that hides the instantiation as if it was a singleton: MyWrapper.Instance is a property that just returns new MyClass();

Only a Sith deals in absolutes

Of course, there are exceptions to my dislike of static methods. True utility classes that do not pose any risk to bloat are excellent cases for static methods - System.Convert as an example. If your project is a one-off with no requirements for future maintenance, the overall architecture really isn't very important - static or non static, doesn't really matter - development speed does, however.

Standards, standards, standards!

Using instance methods does not inhibit you from also using static methods, and vice versa. As long as there's reasoning behind the differentiation and it's standardised. There's nothing worse than looking over a business layer sprawling with different implementation methods.

Has anyone ever noticed that the statement, "Only a Sith deals in absolutes," is an absolute? Sorry. Couldn't help it.
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John KraftOct 27 '08 at 21:44

2

Just as I go out on a limb to get my point across, so does Yegge. Some of his points comes down to normal programming consideration - if the singleton keeps valuable resources open, that may be a problem, of course. Given careful consideration, there are places for singletons and statics alike.
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Mark S. RasmussenOct 27 '08 at 22:16

33

@John Kraft: That was obviously written by a Sith.
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NotMeDec 9 '10 at 19:53

1

I agree, I use statics myself too. In contrast to earlier, I just consider my reasoning before doing so, now :)
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Mark S. RasmussenSep 1 '11 at 7:52

When deciding whether to make a class static or non-static you need to look at what information you are trying to represent. This entails a more 'bottom-up' style of programming where you focus on the data you are representing first. Is the class you are writing a real-world object like a rock, or a chair? These things are physical and have physical attributes such as color, weight which tells you that you may want to instantiate multiple objects with different properties. I may want a black chair AND a red chair at the same time. If you ever need two configurations at the same time then you instantly know you will want to instantiate it as an object so each object can be unique and exist at the same time.

On the other end, static functions tend to lend more to actions which do not belong to a real-world object or an object that you can easily represent. Remember that C#'s predecessors are C++ and C where you can just define global functions that do not exist in a class. This lends more to 'top-down' programming. Static methods can be used for these cases where it doesn't make sense that an 'object' performs the task. By forcing you to use classes this just makes it easier to group related functionality which helps you create more maintainable code.

Most classes can be represented by either static or non-static, but when you are in doubt just go back to your OOP roots and try to think about what you are representing. Is this an object that is performing an action (a car that can speed up, slow down, turn) or something more abstract (like displaying output).

You might have even noticed that IoC is called with a static accessor. Most of the time for me, if you can call static methods on a class, that's all you can do so I mark the class as static for extra clarity.

I've started using static classes when I wish to use functions, rather than classes, as my unit of reuse. Previously, I was all about the evil of static classes. However, learning F# has made me see them in a new light.

What do I mean by this? Well, say when working up some super DRY code, I end up with a bunch of one method classes. I may just pull these methods into a static class and then inject them into dependencies using a delegate. This also plays nicely with my DI container of choice Autofac.

Of course taking a direct dependency on a static method is still usually evil (there are some non-evil uses).

Static classes are very useful and have a place, for example libraries.

The best example I can provide is the .Net Math class, a System namespace static class that contains a library of maths functions.

It is like anything else, use the right tool for the job, and if not anything can be abused.

Blankly dismissing static classes as wrong, don't use them, or saying "there can be only one" or none, is as wrong as over using the them.

C#.Net contains a number of static classes that is uses just like the Math class.

So given the correct implementation they are tremendously useful.

We have a static TimeZone class that contains a number of business related timezone functions, there is no need to create multiple instances of the class so much like the Math class it contains a set of globally accesible TimeZone realated functions (methods) in a static class.

The answer saying "there can be only one" is not talking about the number of static classes, but rather cutely (though rather incorrectly) alluding to the fact that there can only be one instance (wrong, because actually there can only be zero instances)
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Kirk WollJan 5 '13 at 0:48

Ah thanks Kirk, hence my confusion; in any case simply dismissing static or blanketly stating that the singleton pattern is better (as I have seen) is in my opinion taking a very valuable tool out of the toolbox.
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DonJan 7 '13 at 20:17

This is another old but very hot question since OOP kicked in.
There are many reasons to use(or not) a static class, of course and most of them have been covered in the multitude of answers.

I will just add my 2 cents to this, saying that, I make a class static, when this class is something that would be unique in the system and that would really make no sense to have any instances of it in the program. However, I reserve this usage for big classes. I never declare such small classes as in the MSDN example as "static" and, certainly, not classes that are going to be members of other classes.

I also like to note that static methods and static classes are two different things to consider. The main disadvantages mentioned in the accepted answer are for static methods. static classes offer the same flexibility as normal classes(where properties and parameters are concerned), and all methods used in them should be relevant to the purpose of the existence of the class.

A good example, in my opinion, of a candidate for a static class is a "FileProcessing" class, that would contain all methods and properties relevant for the program's various objects to perform complex FileProcessing operations. It hardly has any meaning to have more than one instance of this class and being static will make it readily available to everything in your program.

I'd rather not use extension methods for helper methods. Extension methods are messy, confusing for other developers and not generally intuitive across other languages. Extension methods have their purpose, but not as generic helper methods imho.
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Mark S. RasmussenOct 27 '08 at 21:02

2

Just to clarify, by helper method I mean something like: string.ToSentence(), or string.Camelize(). Essentially anything that would live in a class like StringHelpers.
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jonniiOct 27 '08 at 21:15

Extension methods put you on a slippery slope...I think if you find yourself adding more than 2 extension methods for a given type, it might be time to examine why.
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Jason BuntingOct 27 '08 at 21:45

Those two examples, I would especially not like. The more generic the type, the less suitable it is for extension methods. The worst example is beginning to add ToInt32() method to Object, ToDateTime() etc. I'd much rather have these in separate classes.
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Mark S. RasmussenOct 27 '08 at 21:49

Static class, When you hear 'Static' you know what a static class is.
Static class has Static Methods and Properties which means those properties and methods can be used without instantiating class,

For example if you want to use a DateTime variable to on a seperate class to know the time that your App started , Then if you use a normal class than you will have to create a new instance of it to use it's property unless your property is an static property,

But if you are using a static property then you can simply access it like this